Disclosed is an adjuvant for use in simultaneous polishing of a cationically charged material and an anionically charged material, which forms a adsorption layer on the cationically charged material in order to increase the polishing selectivity of the anionically charged material to cationically charged material, wherein the adjuvant comprises a polyelectrolyte salt containing: (a) a graft type polyelectrolyte that has a weight average molecular weight of 1,000˜20,000 and comprises a backbone and a side chain; and (b) a basic material. CMP (chemical mechanical polishing) slurry comprising the above adjuvant and abrasive particles is also disclosed.